Wind has been used for a long time as a source of power and in recent years it has become very common to use the wind for producing electrical power. In order to do so, the power in the wind is captured by a set of blades (normally two or three) of a wind power plant. The wind captured by the blades causes a shaft connected to the blades to rotate. The shaft is connected to a rotor of a generator, which hence rotates at the same speed as the shaft, or at a multiple of the speed of the shaft in case the rotor is connected to the shaft via a gearbox. The generator then converts the mechanical power provided by the wind into electrical power for delivery to a grid.
In order to optimize the efficiency of a wind turbine generator, it is preferred to use a variable speed generator, wherein the speed of the rotor and hence the shaft and the blades depend on the wind speed. This implies that an optimum operating point for the WTG at various wind speeds must be established. This is done by controlling the torque or power delivered by the generator. The control system in a WTG normally control of the reactive power interchanged between the WTG and the grid as well as the active power drawn from the WTG in order to track the optimum operating point for the WTG. Active power is the component of total, or apparent, electric power that performs work and is measured in watts. The actual work performed by the active power differ it from reactive power which is measured in volt-amperes reactive and establishes and sustains the electric and magnetic fields of alternating current machines. The apparent power, measured in volt-amperes, is the vector sum of the real and reactive power.
Power and torque of a WTG are related by the angular velocity (i.e. the rotational speed of the rotor) according toP=ωrotor·T 
This implies that torque and power control exhibit different characteristics when applied to a WTG. More specifically, when controlling the torque of a WTG, it is necessary to include the rotor speed in the control loop. Power control is hence superior to torque control when the signal to be controlled is power since the transient response is different for the two control methods, i.e. when using torque control, a change in power would require both the rotor speed and the torque to settle before proper control may be applied.
A first type of control systems for WTGs relate to the control of (normally) three 120° spatially displaced sinusoidal voltages which are applied to the three stator phases of the generator. The generation of the sine waves is based on the properties of the generator, i.e. an equivalent model for the generator when operating in its steady state is derived from the electrical and mechanical characteristics of the generator wherein the control system is designed based on the type of generator used (e.g. asynchronous or synchronous).
The generation of one of the sine waves in the three phase system is normally performed independently of the other sine waves, i.e., this type of control systems operates as three separate single phase system controls rather than one common control of a three phase system. This fact results in that any imbalance in the three phase system or any interaction between the phases will not be considered in this type of control. Moreover, it is evident that the generator model will only be valid during steady state operation of the generator. During transient operation of the generator (start, stop, load changes, etc.) the control will hence allow high peak voltage and current transients. This result in a decreased power conversion efficiency as well as a need to oversize the electrical components of the WTG system in order to cope with transient surge currents and voltages.
In order to overcome the drawbacks of the above control structure, an alternative control structure generally named Field Oriented Control (FOC) have been introduced. The main idea behind FOC is to control the stator currents of the generator by using a vector representation of the currents. More specifically, FOC is based on coordinate transformations which transform a three phase time and speed dependent system into a two coordinate time invariant system.
The advantage of performing a transformation from a three phase stationary coordinate system to a rotating coordinate system is that the control of the generator may be done by controlling DC quantities. The transformation is performed in two steps: 1) transformation from the three phase abc stationary coordinate system to a two phase so called αβ stationary coordinate system (known as Clarke transformation), and 2) transformation from the αβ stationary coordinate system to a dq rotating coordinate system (known as Park transformation). More specifically, the transformation from the natural abc reference frame to the synchronous dq reference frame is obtained by the equations
            [                                                  α              u                                                          β              u                                                          0              u                                          ]        =                  [                                                            a                u                                                                    b                u                                                                    c                u                                                    ]            ⁢                        2          3                ⁡                  [                                                    1                                            0                                                              1                  2                                                                                                      -                                      1                    2                                                                                                                    3                                    2                                                                              1                  2                                                                                                      -                                      1                    2                                                                                                -                                                            3                                        2                                                                                                1                  2                                                              ]                                and      ⁢                          [                                                  d              u                                                          q              u                                                          0              u                                          ]        =                  [                                                            α                u                                                                    β                u                                                                    0                u                                                    ]            ⁡              [                                                            cos                ⁢                                                                  ⁢                θ                                                                                      -                  sin                                ⁢                                                                  ⁢                θ                                                    0                                                                          sin                ⁢                                                                  ⁢                θ                                                                    cos                ⁢                                                                  ⁢                θ                                                    0                                                          0                                      0                                      1                                      ]            
which gives
      [                                        d            u                                                q            u                                                0            u                                ]    =            [                                                  a              u                                                          b              u                                                          c              u                                          ]        ⁢                  2        3            ⁡              [                                                            cos                ⁢                                                                  ⁢                θ                                                                                      -                  sin                                ⁢                                                                  ⁢                θ                                                                    1                2                                                                                        cos                ⁡                                  (                                      θ                    -                                                                  2                        ⁢                        π                                            3                                                        )                                                                                    -                                  sin                  ⁡                                      (                                          θ                      -                                                                        2                          ⁢                          π                                                3                                                              )                                                                                                      1                2                                                                                        cos                ⁡                                  (                                      θ                    +                                                                  2                        ⁢                        π                                            3                                                        )                                                                                    -                                  sin                  ⁡                                      (                                          θ                      +                                                                        2                          ⁢                          π                                                3                                                              )                                                                                                      1                2                                                    ]            
where θ=ωt is the angle between the stationary α axis and the synchronous d axis.
Controlling a generator by means of FOC requires the provision of a q axis aligned torque component and a d axis aligned flux component as input to the system. As explained above, the d and q oriented components are transformations from the stationary three phase coordinate system which implies that the FOC, due to the direct coupling to the three phase electrical quantities, will handle both steady state and transient operation of system irrespective of the generator model.
The electromechanical torque TEM of the generator in the dq coordinate system may be expressed asTEM∝Ψrotor·iqstator 
which makes it easy to apply direct torque control in comparison to first type of control system disclosed above. More specifically, by keeping the amplitude of the rotor flux at a fixed value it is possible to control the torque component of the stator current due to the linear relationship between torque and torque component iqstator.
Another technical advantage of FOC compared direct three phase control is the existing level of technical know-how that has been practiced in the DC-drives industry. This leads to a substantial reduction in the design-to-market time of any drive that is controlled using an FOC controller.
U.S. Pat. No. 5,083,039 discloses a variable speed wind turbine comprising a turbine rotor that drives a multiphase generator, a power converter with switches that control stator electrical quantities in each phase of the generator, a torque command device associated with turbine parameter sensors that generates a torque reference signal indicative of a desired torque, and a generator controller operating under field orientation control and responsive to the torque reference signal for defining a desired quadrature axis current and for controlling the switches to produce stator electrical quantities that correspond to the desired quadrature axis current.
Despite the advantages with FOC disclosed above, there are shortcomings of the conventional controllers that the industry has lived with. These constitute e.g. (a) constraint to maintain correct decoupling between the flux and torque producing components of the stator currents during steady state and dynamics, (b) controlling the currents using linear controllers at higher speeds and higher modulation index. Case (a) relates to the parameter sensitivity and the need for adaptation of the same. This may put the controller reliability into stress under extreme conditions of load. Case (b) on the other hand relates to under utilization of the DC-link voltage due to lack of faithful control at higher modulation indexes.
As both these conditions are critical for a high power drive operation from both reliability and cost view point, it is important to provide alternative methods for generator power control.